Hey Storytellers,
Do you have one of those players that obsessively read the monster manuals? They know what the monster is by the beginning of the second sentence of flavor text.
But that isn't the worst part, is it? No, they immediately tell everyone at the table how to fight it, the monster's strengths and weaknesses. This would be fine if they rolled a knowledge roll, but no this person is playing a fighter with no knowledge ranks, yet seemingly their character literally knows everything in the entire pantheon and to defeat it.
How? Well, they are metagaming. They are pulling in outside information that their character cannot know.
Two ways to take care of this: 1) Talk to them outside of the game about it and have your other players watch for it if need be, 2) Change some of the standard monsters to be immune to fire instead of acid if that could make sense or maybe silver doesn't do double damage now cold iron does.